In image forming devices such as an electrophotographic device, an electrostatic recording device and an electrostatic printing device, an image formation method in which an electrostatic latent image formed on a photo conductor is developed with a toner for developing electrostatic images to form a desired image, is widely carried out. This method is applied to a copier, a printer, a facsimile and composite machines thereof.
For example, in the electrophotographic device using electrophotography, in general, the surface of a photoconductor made of a photoconductive substance is uniformly charged by various means, and then an electrostatic latent image is formed on the photoconductor. Subsequently, the electrostatic latent image is developed with a toner, and the toner image is transferred onto a recording material such as a sheet of paper. The recording material is heated so as to fix the image, whereby a copy is obtained.
As for the toner used in an image forming device, in order to improve toner functions (e.g., charge stability and fluidity), external additives such as inorganic and organic particles having a particle diameter lower than that of colored resin particles (toner particles) are generally attached and added onto the surface of toner particles (external addition) and the resulting toner particles are used.
For the toner obtained by using the conventional external additive, it is sometimes hard to obtain a charge rising property at an early stage of printing. Particularly, the charge rising property is easily influenced at a time of use in a severe environment with a low temperature and a low humidity or with a high temperature and a high humidity. As a result, image quality deterioration due to fog is easily caused at the early stage of printing and leads to such a problem that there is an adverse effect on early-stage printing performance.
In the process of continuously printing a large number of sheets, due to mechanical stress in a development device (an increase in the number of times of contact between toner particles by stirring), such a defect that an external additive is buried in and/or liberated (detached) from the toner particle surfaces is easily caused and makes it difficult to impart a stable charging ability (charge stability) to the toner particles over time. As a result, during printing, a reduction in thin line reproducibility and a deterioration in image quality (such as fog) are caused and lead to such a problem that there is an adverse effect on printing durability performance.
Thus, there is a need for toner improvement so that excellent charge rising property is obtained at an early stage of printing even at a time of use in a severe environment; such a defect that the external additive is buried and/or liberated, is not caused even if, in the process of continuously printing a large number of sheets, there is an increase in the number of times of contact between toner particles by stirring in the development device; and the external additive is maintained in an appropriate attached state over time so that a stable charging ability (charge stability) can be imparted to the toner particles. In the case of having a stable charging ability, there is such an advantage that a change in toner conveyance property is small even during a durable printing process.
Patent Document 1 discloses a positively-chargeable toner comprising toner particles and, as external additives, a dry-type fine silica powder and a wet-type fine silica powder in order to provide a toner which allows image density to be stable over a long period of time so that no fog is caused even in the case of forming images at a low printing rate, wherein the dry-type fine silica powder has a positively charged polar group and a hydrophobic group, and the wet-type fine silica powder has a fluorine-containing negatively charged polar group and is surface-treated with a quaternary ammonium salt silane compound.
Patent Document 2 discloses a positively-chargeable toner for developing electrostatic images, comprising colored resin particles and an external additive in order to provide a toner which can impart a stable charging ability and fluidity to toner particles over time, which keeps thin line reproducibility even if a large number of sheets are continuously printed, and which hardly causes image quality deterioration due to fog even in a high temperature and high humidity environment, wherein the toner contains, as external additives, an external additive A in an amount of 0.01 to 0.5 part by weight with respect to 100 parts by weight of colored resin particles, the external additive A being fatty acid alkali metal salt particles or fatty acid alkaline earth metal salt particles having a number average primary particle diameter of 0.1 to 1 μm, and an external additive B in an amount of 0.2 to 2 parts by weight with respect to 100 parts by weight of the colored resin particles, the external additive B being fine spherical silica particles having a number average primary particle diameter of 40 to 200 nm and a sphericity of 1 to 1.3.
However, the toner disclosed in Patent Document 1 cannot strike a balance between toner conveyance amount stability in a normal temperature and normal humidity environment and fog characteristics in a high temperature and high humidity environment. On the other hand, while the toner disclosed in Patent Document 2 has excellent fog characteristics in a high humidity environment, it is insufficient in toner conveyance amount stability in a normal temperature and normal humidity environment.